Heat - How it works

Heat, Work, and Energy

Thermodynamics is the study of the relationships between heat, work,
and energy. Work is the exertion of force over a given distance to
displace or move an object, and is, thus, the product of force and
distance exerted in the same direction. Energy, the ability to
accomplish work, appears in numerous manifestations—including
thermal energy, or the energy associated with heat.

Thermal and other types of energy, including electromagnetic, sound,
chemical, and nuclear energy, can be described in terms of two
extremes: kinetic energy, or the energy associated with movement, and
potential energy, or the energy associated with position. If a spring
is pulled back to its maximum point of tension, its potential energy
is also at a maximum; once it is released and begins springing through
the air to return to its original position, it begins gaining kinetic
energy and losing potential energy.

All manifestations of energy appear in both kinetic and potential
forms, somewhat like the way football teams are organized to play both
offense or defense. Just as a football team takes an offensive role
when it has the ball, and a defensive role when the other team has it,
a physical system typically undergoes regular transformations between
kinetic and potential energy, and may have more of one or the other,
depending on what is taking place in the system.

What Heat Is and Is Not

Thermal energy is actually a form of kinetic energy generated by the
movement of particles at the atomic or molecular level: the greater
the movement of these particles, the greater the thermal energy. Heat
is internal thermal energy that flows from one body of matter to
another—or, more specifically, from a system at a higher
temperature to one at a lower temperature. Thus, temperature, like
heat, requires a scientific definition quite different from its common
meaning: temperature measures the average molecular kinetic energy of
a system, and governs the direction of internal energy flow between
them.

Two systems at the same temperature are said to be in a state of
thermal equilibrium. When this occurs, there is no exchange of heat.
Though in common usage, "heat" is an expression of
relative warmth or coldness, in physical terms, heat exists only in
transfer between two systems. What people really mean by
"heat" is the internal energy of a system—energy
that is a property of that system rather than a property of
transferred internal energy.

I
F YOU HOLD A SNOWBALL IN YOUR HAND
,
AS
V
ANNA
W
HITE AND HER SON ARE DOING IN THIS PICTURE
,
HEAT WILL MOVE FROM YOUR HAND TO THE SNOWBALL
. Y
OUR HAND EXPERIENCES THIS AS A SENSATION OF COLDNESS
. (

Reuters NewMedia Inc./Corbis

.
Reproduced by permission.)

NO SUCH THING AS "COLD."

Though the term "cold" has plenty of meaning in the
everyday world, in physics terminology, it does not. Cold and heat are
analogous to darkness and light: again, darkness means something in
our daily experience, but in physical terms, darkness is simply the
absence of light. To speak of cold or darkness as entities unto
themselves is rather like saying, after spending 20 dollars, "I
have 20 non-dollars in my pocket."

If you grasp a snowball in your hand, of course, your hand gets cold.
The human mind perceives this as a transfer of cold from the snowball,
but, in fact, exactly the opposite happens: heat moves from your hand
to the snow, and if enough heat enters the snowball, it will melt. At
the same time, the departure of heat from your hand results in a loss
of internal energy near the surface of your hand, which you experience
as a sensation of coldness.

Transfers of Heat

In holding the snowball, heat passes from the surface of the hand by
one means, conduction, then passes through the snowball by another
means, convection. In fact, there are three methods heat is
transferred: conduction, involving successive molecular collisions and
the transfer of heat between two bodies in contact; convection, which
requires the motion of fluid from one place to another; or radiation,
which takes place through electromagnetic waves and requires no
physical medium, such as water or air, for the transfer.

CONDUCTION.

Solids, particularly metals, whose molecules are packed relatively
close together, are the best materials for conduction. Molecules of
liquid or nonmetallic solids vary in their ability to conduct heat,
but gas is a poor conductor, because of the loose attractions between
its molecules.

The qualities that make metallic solids good conductors of heat, as a
matter of fact, also make them good conductors of electricity. In the
conduction of heat, kinetic energy is passed from molecule to
molecule, like a long line of people standing shoulder to shoulder,
passing a secret. (And, just as the original phrasing of the secret
becomes garbled, some kinetic energy is inevitably lost in the series
of transfers.)

As for electrical conduction, which takes place in a field of electric
potential, electrons are freed from their atoms; as a result, they are
able to move along the line of molecules. Because plastic is much less
conductive than metal, an electrician uses a screwdriver with a
plastic handle; similarly, a metal cooking pan typically has a wooden
or plastic handle.

CONVECTION.

Wherever fluids are involved—and in physics,
"fluid" refers both to liquids and
gases—convection is a common form of heat transfer. Convection
involves the movement of heated material—whether it is air,
water, or some other fluid.

Convection is of two types: natural convection and forced convection,
in which a pump or other mechanism moves the heated fluid. When heated
air rises, this is an example of natural convection. Hot air has a
lower density than that of the cooler air in the atmosphere above it,
and, therefore, is buoyant; as it rises, however, it loses energy and
cools. This cooled air, now denser than the air around it, sinks
again, creating a repeating cycle that generates wind.

Examples of forced convection include some types of ovens and even a
refrigerator or air conditioner. These two machines both move warm air
from an interior to an exterior place. Thus, the refrigerator pulls
hot air from the compartment and expels it to the surrounding room,
while an air conditioner pulls heat from a building and releases it to
the outside.

But forced convection does not necessarily involve humanmade machines:
the human heart is a pump, and blood carries excess heat generated by
the body to the skin. The heat passes through the skin by means of
conduction, and at the surface of the skin, it is removed from the
body in a number of ways, primarily by the cooling evaporation of
perspiration.

RADIATION.

Outer space, of course, is cold, yet the Sun's rays warm the
Earth, an apparent paradox. Because there is no atmosphere in space,
convection is impossible. In fact, heat from the Sun is not dependant
on any fluid medium for its transfer: it comes to Earth by means of
radiation. This is a form of heat transfer significantly different
from the other two, because it involves electromagnetic energy,
instead of ordinary thermal energy generated by the action of
molecules. Heat from the Sun comes through a relatively narrow area of
the light spectrum, including infrared, visible light, and ultraviolet
rays.

Every form of matter emits electromagnetic waves, though their
presence may not be readily perceived. Thus, when a metal rod is
heated, it experiences conduction, but part of its heat is radiated,
manifested by its glow—visible light. Even when the heat in an
object is not visible, however, it may be radiating electromagnetic
energy, for instance, in the form of infrared light. And, of course,
different types of matter radiate better than others: in general, the
better an object is at receiving radiation, the better it is at
emitting it.

Measuring Heat

The measurement of temperature by degrees in the Fahrenheit or Celsius
scales is a part of everyday life, but measurements of heat are not as
familiar to the average person. Because heat is a form of energy, and
energy is the ability to perform work, heat is, therefore, measured by
the same units as work.

The principal unit of work or energy in the metric system (known
within the scientific community as SI, or the SI system) is the joule.

A
REFRIGERATOR IS A TYPE OF REVERSE HEAT ENGINE THAT USES A
COMPRESSOR
,
LIKE THE ONE SHOWN AT THE BACK OF THIS REFRIGERATOR
,
TO COOL THE REFRIGERATOR
'
S INTERIOR
. (

Ecoscene/Corbis

.
Reproduced by permission.)

Abbreviated "J," a joule is equal to 1 newton-meter (N
· m). The newton is the SI unit of force, and since work is equal
to force multiplied by distance, measures of work can also be
separated into these components. For instance, the British measure of
work brings together a unit of distance, the foot, and a unit of
force, the pound. A foot-pound (ft · lb) is equal to 1.356 J, and
1 joule is equal to 0.7376 ft · lb.

In the British system, Btu, or British thermal unit, is another
measure of energy used for machines such as air conditioners. One Btu
is equal to 778 ft · lb or 1,054 J. The kilocalorie in addition
to the joule, is an important SI measure of heat. The amount of energy
required to change the temperature of 1 gram of water by 1°C is
called a calorie, and a kilocalorie is equal to 1,000 calories.
Somewhat confusing is the fact that the dietary Calorie (capital C),
with which most people are familiar, is not the same as a calorie
(lowercase C)—rather, a dietary Calorie is the equivalent of a
kilocalorie.

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